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Summary: Protected Ctors | Methods | Inherited Methods

# AbstractSet

`public abstract class AbstractSet`
`` `extends AbstractCollection<E>` `implements Set<E>`

java.lang.Object
↳ | java.util.AbstractCollection<E>
| ↳ | java.util.AbstractSet<E>

Known direct subclasses ConcurrentSkipListSet<E>, CopyOnWriteArraySet<E>, EnumSet<E extends Enum<E>>, HashSet<E>, TreeSet<E>
ConcurrentSkipListSet<E> | A scalable concurrent `NavigableSet` implementation based on a `ConcurrentSkipListMap`.
CopyOnWriteArraySet<E> | A `Set` that uses an internal `CopyOnWriteArrayList` for all of its operations.
EnumSet<E extends Enum<E>> | A specialized `Set` implementation for use with enum types.
HashSet<E> | This class implements the `Set` interface, backed by a hash table (actually a `HashMap` instance).
TreeSet<E> | A `NavigableSet` implementation based on a `TreeMap`.

Known indirect subclasses LinkedHashSet<E>
LinkedHashSet<E> | Hash table and linked list implementation of the `Set` interface, with predictable iteration order.

This class provides a skeletal implementation of the `Set` interface to minimize the effort required to implement this interface.

The process of implementing a set by extending this class is identical to that of implementing a Collection by extending AbstractCollection, except that all of the methods and constructors in subclasses of this class must obey the additional constraints imposed by the `Set` interface (for instance, the add method must not permit addition of multiple instances of an object to a set).

Note that this class does not override any of the implementations from the `AbstractCollection` class. It merely adds implementations for `equals` and `hashCode`.

This class is a member of the Java Collections Framework.

See also:

- `Collection`
- `AbstractCollection`
- `Set`

## Summary

### Protected constructors
`AbstractSet()` Sole constructor.

### Public methods
`boolean` | `equals(Object o)` Compares the specified object with this set for equality.
`int` | `hashCode()` Returns the hash code value for this set.
`boolean` | `removeAll(Collection<?> c)` Removes from this set all of its elements that are contained in the specified collection (optional operation).

### Inherited methods
From class `java.util.AbstractCollection`
`boolean` | `add(E e)` Ensures that this collection contains the specified element (optional operation). This implementation always throws an `UnsupportedOperationException`.
`boolean` | `addAll(Collection<? extends E> c)` Adds all of the elements in the specified collection to this collection (optional operation). This implementation iterates over the specified collection, and adds each object returned by the iterator to this collection, in turn.
`void` | `clear()` Removes all of the elements from this collection (optional operation). This implementation iterates over this collection, removing each element using the `Iterator.remove` operation.
`boolean` | `contains(Object o)` Returns `true` if this collection contains the specified element. This implementation iterates over the elements in the collection, checking each element in turn for equality with the specified element.
`boolean` | `containsAll(Collection<?> c)` Returns `true` if this collection contains all of the elements in the specified collection. This implementation iterates over the specified collection, checking each element returned by the iterator in turn to see if it's contained in this collection.
`boolean` | `isEmpty()` Returns `true` if this collection contains no elements. This implementation returns `size() == 0`.
`abstract Iterator<E>` | `iterator()` Returns an iterator over the elements contained in this collection.
`boolean` | `remove(Object o)` Removes a single instance of the specified element from this collection, if it is present (optional operation). This implementation iterates over the collection looking for the specified element.
`boolean` | `removeAll(Collection<?> c)` Removes all of this collection's elements that are also contained in the specified collection (optional operation). This implementation iterates over this collection, checking each element returned by the iterator in turn to see if it's contained in the specified collection.
`boolean` | `retainAll(Collection<?> c)` Retains only the elements in this collection that are contained in the specified collection (optional operation). This implementation iterates over this collection, checking each element returned by the iterator in turn to see if it's contained in the specified collection.
`abstract int` | `size()` Returns the number of elements in this collection.
`<T> T[]` | `toArray(T[] a)` Returns an array containing all of the elements in this collection; the runtime type of the returned array is that of the specified array. This implementation returns an array containing all the elements returned by this collection's iterator in the same order, stored in consecutive elements of the array, starting with index `0`.
`Object[]` | `toArray()` Returns an array containing all of the elements in this collection. This implementation returns an array containing all the elements returned by this collection's iterator, in the same order, stored in consecutive elements of the array, starting with index `0`.
`String` | `toString()` Returns a string representation of this collection.
From class `java.lang.Object`
`Object` | `clone()` Creates and returns a copy of this object.
`boolean` | `equals(Object obj)` Indicates whether some other object is "equal to" this one.
`void` | `finalize()` Called by the garbage collector on an object when garbage collection determines that there are no more references to the object.
`final Class<?>` | `getClass()` Returns the runtime class of this `Object`.
`int` | `hashCode()` Returns a hash code value for the object.
`final void` | `notify()` Wakes up a single thread that is waiting on this object's monitor.
`final void` | `notifyAll()` Wakes up all threads that are waiting on this object's monitor.
`String` | `toString()` Returns a string representation of the object.
`final void` | `wait(long timeout, int nanos)` Causes the current thread to wait until another thread invokes the `notify()` method or the `notifyAll()` method for this object, or some other thread interrupts the current thread, or a certain amount of real time has elapsed.
`final void` | `wait(long timeout)` Causes the current thread to wait until either another thread invokes the `notify()` method or the `notifyAll()` method for this object, or a specified amount of time has elapsed.
`final void` | `wait()` Causes the current thread to wait until another thread invokes the `notify()` method or the `notifyAll()` method for this object.
From interface `java.util.Collection`
`abstract boolean` | `add(E e)` Ensures that this collection contains the specified element (optional operation).
`abstract boolean` | `addAll(Collection<? extends E> c)` Adds all of the elements in the specified collection to this collection (optional operation).
`abstract void` | `clear()` Removes all of the elements from this collection (optional operation).
`abstract boolean` | `contains(Object o)` Returns `true` if this collection contains the specified element.
`abstract boolean` | `containsAll(Collection<?> c)` Returns `true` if this collection contains all of the elements in the specified collection.
`abstract boolean` | `equals(Object o)` Compares the specified object with this collection for equality.
`abstract int` | `hashCode()` Returns the hash code value for this collection.
`abstract boolean` | `isEmpty()` Returns `true` if this collection contains no elements.
`abstract Iterator<E>` | `iterator()` Returns an iterator over the elements in this collection.
`default Stream<E>` | `parallelStream()` Returns a possibly parallel `Stream` with this collection as its source.
`abstract boolean` | `remove(Object o)` Removes a single instance of the specified element from this collection, if it is present (optional operation).
`abstract boolean` | `removeAll(Collection<?> c)` Removes all of this collection's elements that are also contained in the specified collection (optional operation).
`default boolean` | `removeIf(Predicate<? super E> filter)` Removes all of the elements of this collection that satisfy the given predicate.
`abstract boolean` | `retainAll(Collection<?> c)` Retains only the elements in this collection that are contained in the specified collection (optional operation).
`abstract int` | `size()` Returns the number of elements in this collection.
`default Spliterator<E>` | `spliterator()` Creates a `Spliterator` over the elements in this collection.
`default Stream<E>` | `stream()` Returns a sequential `Stream` with this collection as its source.
`abstract <T> T[]` | `toArray(T[] a)` Returns an array containing all of the elements in this collection; the runtime type of the returned array is that of the specified array.
`abstract Object[]` | `toArray()` Returns an array containing all of the elements in this collection.
From interface `java.util.Set`
`abstract boolean` | `add(E e)` Adds the specified element to this set if it is not already present (optional operation).
`abstract boolean` | `addAll(Collection<? extends E> c)` Adds all of the elements in the specified collection to this set if they're not already present (optional operation).
`abstract void` | `clear()` Removes all of the elements from this set (optional operation).
`abstract boolean` | `contains(Object o)` Returns `true` if this set contains the specified element.
`abstract boolean` | `containsAll(Collection<?> c)` Returns `true` if this set contains all of the elements of the specified collection.
`abstract boolean` | `equals(Object o)` Compares the specified object with this set for equality.
`abstract int` | `hashCode()` Returns the hash code value for this set.
`abstract boolean` | `isEmpty()` Returns `true` if this set contains no elements.
`abstract Iterator<E>` | `iterator()` Returns an iterator over the elements in this set.
`static <E> Set<E>` | `of(E e1, E e2, E e3)` Returns an immutable set containing three elements.
`static <E> Set<E>` | `of(E e1, E e2, E e3, E e4, E e5, E e6, E e7, E e8, E e9)` Returns an immutable set containing nine elements.
`static <E> Set<E>` | `of(E e1, E e2, E e3, E e4, E e5)` Returns an immutable set containing five elements.
`static <E> Set<E>` | `of(E e1, E e2, E e3, E e4, E e5, E e6)` Returns an immutable set containing six elements.
`static <E> Set<E>` | `of(E e1, E e2, E e3, E e4, E e5, E e6, E e7, E e8)` Returns an immutable set containing eight elements.
`static <E> Set<E>` | `of()` Returns an immutable set containing zero elements.
`static <E> Set<E>` | `of(E e1, E e2, E e3, E e4, E e5, E e6, E e7, E e8, E e9, E e10)` Returns an immutable set containing ten elements.
`static <E> Set<E>` | `of(E e1, E e2)` Returns an immutable set containing two elements.
`static <E> Set<E>` | `of(E... elements)` Returns an immutable set containing an arbitrary number of elements.
`static <E> Set<E>` | `of(E e1, E e2, E e3, E e4, E e5, E e6, E e7)` Returns an immutable set containing seven elements.
`static <E> Set<E>` | `of(E e1, E e2, E e3, E e4)` Returns an immutable set containing four elements.
`static <E> Set<E>` | `of(E e1)` Returns an immutable set containing one element.
`abstract boolean` | `remove(Object o)` Removes the specified element from this set if it is present (optional operation).
`abstract boolean` | `removeAll(Collection<?> c)` Removes from this set all of its elements that are contained in the specified collection (optional operation).
`abstract boolean` | `retainAll(Collection<?> c)` Retains only the elements in this set that are contained in the specified collection (optional operation).
`abstract int` | `size()` Returns the number of elements in this set (its cardinality).
`default Spliterator<E>` | `spliterator()` Creates a `Spliterator` over the elements in this set.
`abstract Object[]` | `toArray()` Returns an array containing all of the elements in this set.
`abstract <T> T[]` | `toArray(T[] a)` Returns an array containing all of the elements in this set; the runtime type of the returned array is that of the specified array.
From interface `java.lang.Iterable`
`default void` | `forEach(Consumer<? super T> action)` Performs the given action for each element of the `Iterable` until all elements have been processed or the action throws an exception.
`abstract Iterator<E>` | `iterator()` Returns an iterator over elements of type `T`.
`default Spliterator<E>` | `spliterator()` Creates a `Spliterator` over the elements described by this `Iterable`.

## Protected constructors

### AbstractSet

    protected AbstractSet ()

Sole constructor. (For invocation by subclass constructors, typically implicit.)

## Public methods

### equals

    public boolean equals (Object o)

Compares the specified object with this set for equality. Returns `true` if the given object is also a set, the two sets have the same size, and every member of the given set is contained in this set. This ensures that the `equals` method works properly across different implementations of the `Set` interface.

This implementation first checks if the specified object is this set; if so it returns `true`. Then, it checks if the specified object is a set whose size is identical to the size of this set; if not, it returns false. If so, it returns `containsAll((Collection) o)`.

Parameters
`o` | `Object`: object to be compared for equality with this set

Returns
`boolean` | `true` if the specified object is equal to this set

### hashCode

    public int hashCode ()

Returns the hash code value for this set. The hash code of a set is defined to be the sum of the hash codes of the elements in the set, where the hash code of a `null` element is defined to be zero. This ensures that `s1.equals(s2)` implies that `s1.hashCode()==s2.hashCode()` for any two sets `s1` and `s2`, as required by the general contract of `Object#hashCode`.

This implementation iterates over the set, calling the `hashCode` method on each element in the set, and adding up the results.

Returns
`int` | the hash code value for this set

See also:

- `Object.equals(Object)`
- `Set.equals(Object)`

### removeAll

    public boolean removeAll (Collection<?> c)

Removes from this set all of its elements that are contained in the specified collection (optional operation). If the specified collection is also a set, this operation effectively modifies this set so that its value is the asymmetric set difference of the two sets.

This implementation determines which is the smaller of this set and the specified collection, by invoking the `size` method on each. If this set has fewer elements, then the implementation iterates over this set, checking each element returned by the iterator in turn to see if it is contained in the specified collection. If it is so contained, it is removed from this set with the iterator's `remove` method. If the specified collection has fewer elements, then the implementation iterates over the specified collection, removing from this set each element returned by the iterator, using this set's `remove` method.

Note that this implementation will throw an `UnsupportedOperationException` if the iterator returned by the `iterator` method does not implement the `remove` method.

Parameters
`c` | `Collection`: collection containing elements to be removed from this set

Returns
`boolean` | `true` if this set changed as a result of the call

Throws
`UnsupportedOperationException` | if the `removeAll` operation is not supported by this set
`ClassCastException` | if the class of an element of this set is incompatible with the specified collection (optional)
`NullPointerException` | if this set contains a null element and the specified collection does not permit null elements (optional), or if the specified collection is null

See also:

- `AbstractCollection.remove(Object)`
- `AbstractCollection.contains(Object)`

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